Merge headers from nitro decomp (#69)

* merge headers from nitro decomp

* cleanup: use inlines from g2.h and gx.h when changing registers

* cleanup: remove externs declarations in source files

include/input.hpp

@@ -0,0 +1,19 @@
+#pragma once
+
+#include "types.h"
+
+#include <nitro/pad.h>
+
+#define CHECK_BUTTON_COMBO(value, btn) ((value) & (btn))
+
+struct Input {
+    /* 00 */ u16 cur;
+    /* 02 */ volatile u16 press;
+    /* 04 */ u16 release;
+    /* 06 */
+
+    Input();
+    unk32 func_02013c08(u16 param1);
+    unk32 func_02013b24(unk32 param1);
+    unk32 func_02013bbc();
+};